This disclosure relates generally to an apparatus and method for simultaneously determining an IR spectrum of multiple sample materials. More particularly, various embodiments of the disclosure relates to spatial multiplexing of spectroscopically determined IR spectra of multiple samples using an apparatus and method that operate in real-time with simultaneous background compensation, and which do not require the use of any moving parts. Still further, the apparatus and method of the disclosure do not require extensive mathematical transformation of the detected spectral information to analyze the composition of the sample material.
This disclosure has industrial applicability to, for example, a real-time method to monitor manufacturing processes. Such processes include, but are not limited to measurement of thickness, chemical structure, and orientation of coatings on surfaces (solid, liquid, chemically bound, physically adsorbed). These measurements include, but are not limited to those made on biological materials, polymers, superconductors, semiconductors, metals, dielectrics, and minerals. Further applicability is found to a real-time apparatus and method to measure and detect a chemical species present in a chemical reaction involving various processing of materials in any of a gaseous, liquid, or solid state. In addition, the apparatus and method of this disclosure provides for self-compensation, to account for sensor or optical path changes over time, or changes in environmental conditions, which may affect the measurements obtained.
As industry continues on its path of cost reductions in core technologies, more emphasis will be placed on the optimization of processes and performance. This retrenchment will necessitate the development and introduction of a whole new class of sophisticated instrumentation that is portable, rugged, reliable, and capable of operation over long periods of time in an aggressive industrial or other non-laboratory environment.
Spectrometric techniques are often used in analysis of materials. Classically, spectroscopy is the measurement of the selective absorption, emission, or scattering of light (energy) of specific colors by matter. Visible white light can be separated into its component colors, or spectrum, by a prism, for example. The principal purpose of a spectroscopic measurement is usually to identify the chemical composition of an unknown material, or to elucidate details of the structure, motion, or environmental characteristics (e.g., internal temperature, pressure, magnetic field strength, etc.) of a “known” material or object. Spectroscopy's widespread technical importance to many areas of science and industry can be traced back to nineteenth-century successes, such as characterizing natural and synthetic dyes, and determining the elemental compositions of stars.
Modern applications of spectroscopy have generalized the meaning of “light” to include the entire range or spectrum of electromagnetic radiation, which extends from gamma-and x-rays, through ultraviolet, visible, and infrared light, to microwaves and radio waves. All these various forms (or wavelength ranges) of electromagnetic radiation have their own characteristic methods of measurement. These different methods give rise to various types of spectroscopic apparatus and techniques that are outwardly very different from each other, and which often rely upon difference physical phenomena to make measurements of material characteristics. Further, the various experts and other researchers in these diverse fields, more often than not, do not cross the technical boundaries between these areas of specialization, as different and somewhat compartmentalized knowledge bases and “rules of thumb” are used.
The use of infrared (IR) is one of numerous spectroscopic techniques for analyzing the chemistry of materials. In all cases, spectroscopic analysis implies a measurement of a very specific wavelength of light energy, either in terms of the amount absorbed or reflected by the sample in question, or the amount emitted from the sample when suitably energized.
In the case of IR, an absorption form of spectrometric analysis is relied upon. IR radiation does not have enough energy to induce transitions between different electronic states, i.e., between molecular orbitals, as seen with ultraviolet (UV), for example. Unlike atomic absorption, IR spectroscopy examines vibrational transitions within a single electronic state of a molecule, and is not concerned with specific atomic elements, such as Pb, Cu, etc. Such vibrations fall into one of three main categories, i.e., stretching, which results from a change in inter-atomic distance along the bond axis; bending, which results from a change in the angle between two bonds; and torsional coupling, which relates to a change in angle and separation distance between two groups of atoms. Almost all materials absorb IR radiation, except homonuclear diatomic molecules, e.g., O2, H2, N2, Cl2, F2, or noble gases.
IR typically covers the range of the electromagnetic spectrum between 0.78 and 1000 μm. Within the context of IR spectroscopy, temporal frequencies are measured in “wavenumbers” (in units of cm−1), which are calculated by taking the reciprocal of the wavelength (in centimeters) of the radiation. Although not precisely defined, the IR range is sometimes further delineated by three regions having the wavelength and corresponding wavenumber ranges indicated:
“near-IR”: 0.78-2.5 μm 12800-4000 cm−1;
“mid-IR” 2.5-50 μm 4000-200 cm−1; and
“far-IR” 50-1000 μm 200-10 cm−1 
For a molecule to absorb IR, the vibrations or rotations within the molecule must cause a net change in the dipole moment of the molecule. The alternating electric field of the incident IR radiation interacts with fluctuations in the dipole moment of the molecule and, if the frequency of the radiation matches the vibrational frequency of the molecule, then radiation will be absorbed, causing a reduction in the IR band intensity due to the molecular vibration.
An electronic state of a molecular functional group may have many associated vibrational states, each at a different energy level. Consequently, IR spectroscopy is concerned with the groupings of atoms in specific chemical combinations to form what are known as “functional groups”, or molecular species. These various functional groups help to determine a material's properties or expected behavior by the absorption characteristics of associated types of chemical bonds. These chemical bonds undergo a change in dipole moment during a vibration. Examples of such functional groups and their respective energy bands include, for example, hydroxyl (O—H) (3610-3640 cm−1), amines (N—H) (3300-3500 cm−1), aromatic rings (C—H) (3000-3100 cm−1), alkenes (C—H) (3020-3080 cm−1), alkanes (C—H) (2850-2960 cm−1), nitriles (C═N) (2210-2260 cm−1), carbonyl (C═O) (1650-1750 cm−1), or amines (C—N) (1180-1360 cm−1). The IR absorption bands associated with each of these functional groups act as a type of “fingerprint” which is very useful in composition analysis, particularly for identification of organic and organometallic molecules.
By knowing which wavelengths are absorbed by each functional group of interest, an appropriate wavelength can be directed at the sample being analyzed, and then the amount of energy absorbed by the sample can be measured. The intensity of the absorption is related to the concentration of the component. The more energy that is absorbed, the more of that particular functional group exists in the sample. Results can therefore be numerically quantified. Further, the absence of an absorption band in a sample can often provide equally useful information.
Intensity and frequency of sample absorption are depicted in a two-dimensional plot called a spectrum. Intensity is generally reported in terms of absorbance, the amount of light absorbed by a sample, or percent transmittance, the amount of light that passes through it. In IR spectroscopy, frequency is usually reported in terms of wavenumbers, as defined above.
Infrared spectrometers may be built using a light source (e.g., the sun), a wavelength discriminating unit or optically dispersive element such as a prism, for example, and a detector sensitive to IR. By scanning the optically dispersive element, spectral information may be obtained at different wavelengths, by using either a reflection mode, i.e., reflection of the light source off the sample, or a transmission mode, i.e., transmitting a portion of the light source through the sample. However, one drawback to this approach is the moving parts associated with the required scanning operation. Such moving parts inherently limit the ruggedness and portability, for example, of such a device.
More recently, a Michelson interferometer has been used to generate a so-called interferogram in the IR spectrum, which later is subjected to Fourier transform processing such as a fast Fourier transform (FFT) to yield the final spectrum. In the IR range, such spectrometers are called FTIR interferometers, and the first commercially available appeared in the mid 1960's. A representation of an FTIR interferometer is provided in FIG. 1.
The key components of FTIR interferometer 100 are IR source 110, interferometer (130, 140, 150), and IR detector 160. FTIR interferometer 100 provides a means for the spectrometer to measure all optical frequencies transmitted through sample 120 simultaneously, modulating the intensity of individual frequencies of radiation before detector 160 picks up the signal. Typically, moving mirror arrangement 150 is used to obtain a path length difference between two (initially) identical beams of light. After traveling a different distance than a reference beam, the second beam and the reference beam are recombined, and an interference pattern results. IR detector 160 is used to detect this interference pattern.
The detected interference pattern, or interferogram, is a plot of intensity versus mirror position. The interferogram is a summation of all the wavelengths emitted by the sample and, for all practical purposes, the interferogram cannot be interpreted in its original form. Using the mathematical process of Fourier Transform (FT), a computer or dedicated processor converts the interferogram into a spectrum that is characteristic of the light either absorbed or transmitted through sample 120.
The development of FT spectroscopy has proven to be one of the most important advances in modern instrumentation development in the 20th Century. Optical spectroscopy utilizing the interference of light has made fast, sensitive detection of molecular vibration/rotation possible due to the large throughput and multiplex advantages provided by FT instrumentation. In Nuclear Magnetic Resonance (NMR) and mass spectroscopy where high-resolution spectra are required, FT instrumentation has also prevailed as the state of the art.
The same technological innovations that have made FT instruments those of choice for a generation of spectroscopists, however, have also made them extremely sensitive to their operating environment. For these reasons, FT interferometers are mostly limited to laboratory conditions which require the use of an optical bench to prevent vibration, and which also require stringent environmental controls to control temperature variations that adversely affect the interferogram by thermally inducing path length differences. While this type of scanning approach has proven to be workable, the signal-to-noise-ratios (SNR) obtainable in some situations often require substantial signal averaging of multiple interferograms, thus making FTIR systems inherently slower than desired under some circumstances, with reduced speed and potentially lower reliability resulting from the numerous moving parts of these systems.
In spectroscopy, resolution is a measure of the ability to resolve or differentiate two peaks in the spectrum, where high resolution corresponds to a small wavenumber difference between the peak positions, and low resolution is associated with a larger wavenumber difference between the peak positions. Fourier Transform interferometers are capable of extremely high resolution, on the order of 1/1000th cm−1, depending on the amount of possible movement of the mirror, or the path length difference that can be generated by the particular apparatus. “Low” resolution is generally considered to be in the range of 16-32 cm−1, although no bright-line demarcation between “low” and “high” resolution exists, as resolution is chosen based on the required measurement and specific application. For typical chemical analysis and identification associated with FTIR, “high” resolution of 8 cm−1 or better is common. Otherwise, chemical information is lost if the resolution is too low, as adjacent peaks identified with a particular chemical bond or vibration state may be “smeared” together and rendered indiscernible if a lower resolution is used.
The need for thermal stability, mechanical vibration isolation, and stringent optical alignment has put severe constraints on where and how FT instruments can be used and, in particular, has limited the portability of such instruments. If discussion is limited to FTIR interferometers, then an examination of the specific technology used in currently available instruments reveals where some of the shortcomings can be found. Table 1 compares the four most commonly used techniques for the operation of an optical interferometer, and their limitations.
TABLE 1Common FTIR Interferometer Designs and their LimitationsOperating TechnologiesLimitationsAir-BearingsRequires stable supply of clean, dry air and atightly leveled travel plane for the movingmirror. Low tolerance for vibration.Magnetic CoilsRequires highly regulated power supplies. Lowtolerance for vibration.Piezo StacksLimited travel range. High voltage powersupplies needed to operate the piezo elements.Mechanical/Piezo HybridRequires large mechanical structures and com-plicated feedback system for piezo elementoperation.
FTIR has been applied to a variety of studies in industry, government, and academic laboratories, and has resulted in a major improvement upon conventional methods of performing analysis on a variety of samples. However, it has become clear that the moving mirror mechanism in a traditional interferometer has limited the design and construction of a more compact and portable FTIR. One potential solution attempted by Stelzle, Tuchtenhagen, and Rabolt (“Novel All-fibre-optic Fourier-transform Spectrometer with Thermally Scanned Interferometer”), was to construct an all-fiber-optic FT Spectrometer, which had no moving parts, and which was used to perform infrared spectroscopy.
In this feasibility study, an attempt was made to build an interferometer in the near-IR (10000-5000 cm−1) range using fiber optics. Two carefully measured and cleaved optical fibers were used as the two light channels, or optical paths, with one fiber kept at ambient temperature while the other fiber was heated/cooled repeatedly. The resulting optical path difference (OPD) between the two fiber channels due to changes in both the length and the refractive index of the heated/cooled fiber caused interference in the combined channel. The heating/cooling cycle was used to generate an OPD of 3 cm, thus producing an interferogram with the power spectrum calculated accordingly.
However, the interference of two light beams in the optical fibers under different thermal and mechanical conditions turned out to be very complex. In contrast to the traditional Michelson interferometer, whose only source of optical path length difference comes from the geometric path length resulting from the moving mirror, a fiber-optic interferometer responds to any mechanical or thermal changes of the operating environment, which causes a scrambling or loss of the phase information necessary for interference to occur. It was concluded that although the fiber optics concept is a good one, a more prudent plan for a no-moving parts IR instrument had to be developed.
In surveying the literature, it became apparent that, without regard to the band of interest, e.g., visible, near-IR, or IR, other approaches to the construction of an FT interferometer with no-moving parts had also been attempted, as depicted in FIG. 2. Such approaches used either a linear array detector or a focal plane array (FPA) to collect interferograms. These designs involved the projection of the center portion of the interferogram onto the detector, and then used the “imaged” interferograms to calculate the power spectra after Fourier Transform processing. One difficulty of these conventional techniques is that the array detector size, its dynamic range, and the limited range of spectral response available limited the range of the interferograms that could be captured by the array detector.
In addition, even without moving parts, these approaches still rely upon calculation-intensive Fourier Transform processing to derive the power spectrum. Hence, there is still a need for a rugged, non-interferometric, no-moving part spectrometer in the mid-IR range.
Aside from, and even prior to Fourier Transform spectroscopy, spectroscopy based on dispersion provided a possible implementation. In this approach, an optically dispersive element, such as a prism or diffraction grating, is used to separate the spectral frequencies present in the incident light radiation. The dispersive element was then rotated, in order to allow the various wavelengths present in the incident light to be detected.
IR spectroscopy based on dispersion became obsolete in most analytical applications in the late 1960's due to its slow scan rate and lower sensitivity. It is well known that the scanning mechanism in a dispersive spectrometer, e.g., a moving prism, intrinsically limits both its ruggedness and optical throughput. The need for scanning comes from the fact that point detection of photons was the only available method at that time, and this was especially true in the IR range of the spectrum. Today, however, array detectors in the visible and near-IR range are widely available for area detection of photons. Charge-coupled-devices (CCD) capable of >80% quantum efficiency (QE) in the visible range have been made and utilized in many applications, such as the visible/near-IR camera aboard the Hubble Space telescope. As a result of this progress, CCD-based high performance spectrograph systems in the visible and near-infrared range can now be purchased through commercial suppliers. These systems provide alternatives to traditional FT interferometers.
However, the range of scientific problems which could now benefit from IR investigations has increased significantly, and applications involving samples which may change their position in the beam (e.g., vibrate or oscillate) while the spectrum is being recorded can not be routinely addressed using conventional FTIR instruments. The scanning architecture of FTIR instruments and the resulting modulation of the different optical frequency components can become modified further by a sample whose position fluctuates, and this can render the spectral information useless.
For example, few techniques exist which can provide insitu structural information about Langmuir films. Infrared reflectance-absorbance spectroscopy (IRRAS) is a non-destructive technique that provides direct structural information about either the expanded or condensed phase of a Langmuir monolayer. The technique can also provide information about both the hydrocarbon tails and the head groups independently by monitoring vibrational modes with frequencies in the 4000 to 400 cm−1 region. Because polarized infrared spectroscopic measurements are sensitive to the orientation of transition-dipole moments, IR-RAS can be used to determine the orientation of different sub-components of an amphiphilic molecule.
Since, in order to fabricate Langmuir-Blodgett (LB) films, Langmuir monolayers of these amphiphilic polymers must be first formed on a water surface, where their thermodynamic state of order is known to have a dramatic effect on the structure of the transferred LB films. Hence, it becomes critically important to understand the structure of the monolayer in situ on the water surface under conditions for which the thermodynamics are well understood. One of these conditions is the continuous compression of the Langmuir monolayer film since, in general, the most accurate and reproducible thermodynamic measurements have been obtained during this process. However, to date, pressure-dependent IRRAS spectra have been collected exclusively in a “step-wise” manner, i.e., no IRRAS spectra have been reported that correspond to a Langmuir mono-layer undergoing a continuous compression.
While IRRAS using conventional FTIR spectroscopy offers a variety of instrumental advantages for investigating thin films compared to standard transmission measurements, the technique does suffer from several inherent limitations. The inherently weak monolayer absorbance bands result in a relatively poor signal-to-noise (S/N) spectrum and, since environmental fluctuations are difficult to minimize, spectral compensation for the water vapor that is present above the Langmuir trough remains a challenge.
Over the last decade the S/N observed in IRRAS experiments on dielectric substrates has gradually improved due to advances in the instrumentation and in the optical interface. There are a variety of ways to minimize the problem of water vapor compensation, including strict humidity control and a shuttle transport system that allows a sample trough to be repeatedly replaced with a reference trough allowing both a sample and reference spectrum to be recorded.
Another way to minimize the problem of water vapor compensation includes the application of polarization modulation infrared reflectance-absorbance spectroscopy (PM-IRRAS). In PM-IRRAS, the polarization of the incident beam undergoes a fast modulation between two orthogonal directions via a photoelastic modulator. The detected signal passes through a two-channel electronic system and is mathematically processed to give a differential reflectivity spectrum. In theory, because of the fast polarization modulation, the PM-IRRAS signal is devoid of all polarization-independent signals such as strong water vapor absorptions, instrumental drifts and fluctuations.
Despite the previously mentioned limitations, the IRRAS or PM-IRRAS technique has been used to investigate a variety of Langmuir monolayers, including studies of fatty acid, phospholipid and phospholipid-protein monolayers. The technique has been used to provide information on lipid conformation, molecular tilt angle, and the structure of head groups, as well as protein secondary structure and orientation. However, neither IRRAS nor PM-IRRAS (both of which utilize FTIR) have been able to provide in situ timeresolved measurements of Langmuir monolayers in the 1 ms to 1 s time regime, nor have any of the known techniques been able to simultaneously provide multiple independent measurements.
Hence, the need for a non-scanning instrument with convenient delivery and detection of IR radiation could never be stronger. For example, applications requiring on-line studies of micro mechanical deformation in polymer thin films during processing, in situ structural studies of aging in Light Emitting Diodes (LEDs), and the monitoring of in-organic (silicon, SiN, etc.) thin film growth on flexible polymer substrates would all benefit from an IR instrument with no moving parts, which as a consequence, will also be robust and portable. Such a portable instrument would facilitate materials research by providing a powerful new tool for thin film studies, especially those with fluctuating sampling geometries or in a remote sample location.
Further advantages for such a non-scanning, real-time instrument in the IR range could be found in environmental monitoring, including monitoring near military or civilian personnel during potential chemical or biological warfare attacks. The complex chemical compositions in such agents show strong IR absorbance, and thus could be readily identified.
In spite of the inroads made in spectroscopy by spectrographs in the visible and near-infrared range, primarily due to the progress in CCD detectors mentioned previously, FT instrumentation still remains dominant in spectroscopy in the mid to far-infrared range and, therefore, instruments in this range are still extremely limited by the operating environment of the interferometer.
Further, all spectral techniques require the collection of a reference spectrum for comparison with that obtained from the sample. In almost all cases, these two measurements are done in series, basically doubling the time of measurement. If this time is long, as in the case of obtaining spectra of thin films or of molecules in the gas phase, then variations in the instrument or sample conditions, for example, due to temperature or humidity fluctuations in the instrument or environment, can prevent compensation of the instrumental background.
Thus, making background and sample measurements in parallel removes or compensates for any instrumental fluctuations, reduces the total time of spectral collection, preserves sample integrity in case there is “aging” or degradation with time, and provides additional advantages in the portability of such an instrument since “realtime” background compensation in aggressive field or non-laboratory environments can be made.
What is needed, then, is a robust, compact, and portable instrument (with no moving parts) in the IR range to address specific applications where sample fluctuations cause significant deterioration of the signal-to-noise ratio in conventional FTIR spectra.
What is further needed is a portable and reliable IR spectroscope which allows multiple, simultaneous spectral measurements.
Still what is further needed is a real-time, sensitive and relatively high-resolution apparatus and method for IR spectroscopic materials analysis, which does not rely upon interferometric or a calculation-intensive Fourier Transform approach, and which is relatively insensitive to harsh environments, including high vibration and wide temperature variations, and which provides the ability to compensate for background spectral components and component degradation in real-time.
There is, therefore, also a need for an apparatus and device capable of collecting multiple independent spectra simultaneously with background environment compensation and compensation for the aging of components, including orthogonally polarized measurements, which allows time-resolved measurements on Langmuir monolayers, including time-resolved molecular orientation measurements.